linux-kernel - Re: [PATCH 1/5] Skip timer works.patch

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Date:	Fri, 27 Oct 2006 14:16:12 -0700
From:	Andi Kleen <ak@...e.de>
To:	virtualization@...ts.osdl.org
Cc:	Zachary Amsden <zach@...are.com>, Andi Kleen <ak@....de>,
	Andrew Morton <akpm@...l.org>,
	Chris Wright <chrisw@...s-sol.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/5] Skip timer works.patch

On Friday 27 October 2006 12:09, Zachary Amsden wrote:
> Andi Kleen wrote:
> > On Thu, Oct 19, 2006 at 05:09:22PM -0700, Zachary Amsden wrote:
> >> Add a way to disable the timer IRQ routing check via a boot option.  The
> >> VMI timer code uses this to avoid triggering the pester Mingo code,
> >> which probes for some very unusual and broken motherboard routings.  It
> >> fires 100% of the time when using a paravirtual delay mechanism instead
> >> of using a realtime delay, since there is no elapsed real time, and the
> >> 4 timer IRQs have not yet been delivered.
> >
> > You mean paravirtualized udelay will not actually wait?
>
> Yes, but even putting that problem aside, the timing element here is
> tricky to get right in a VM.
>
> > This implies that you can't ever use any real timer in that kind of
> > guest, right?
>
> No.  You can use a real timer just fine.  But there is no reason ever to
> use udelay to busy wait for "hardware" in a virtual machine.  Drivers
> which are used for real hardware may turn udelay back on selectively;
> but this is another patch.
>
> >> In addition, it is entirely possible, though improbable, that this bug
> >> could surface on real hardware which picks a particularly bad time to
> >> enter SMM mode, causing a long latency during one of the timer IRQs.
> >
> > We already have a no timer check option. But:
>
> Really?  I didn't see one that disabled the broken motherboard detection
> / workaround code, which is what we are trying to avoid here.

no_timer_check. But it's only there on x86-64 in mainline - although there
were some patches to add it to i386 too.

> That is what this patch is building towards, but the boot option is
> "free", so why not?  In the meantime, it helps non-paravirt kernels
> booted in a VM.

Hmm, you meant they paniced before?  If they just fail a few tests
that is not particularly worrying (real hardware does that often too)

-Andi
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